There is a chemical waste disposal point at Loch Clash Stopover Campervan Site. Cost £5 - payable at the Spar shop. Included in the price if you are stopping overnight.
Please do not put toilet waste into the public toilets. It doesn't matter whether you use green or blue chemicals.
Please do not dispose of grey water anywhere other than a proper disposal point. It is against the law in Scotland to discharge grey water into the ground or even into a storm drain.